Many methods are currently in use which involve the generation of seismic waves at the surface of the earth and the recording of seismic waves at the surface after they have traveled downwardly into the earth, encountered subsurface anomalies of various kinds and then have been deflected upwardly to the surface as by reflection, refraction, and diffraction. Numerous methods of generating the seismic waves are in use. So also are numerous methods for recording the seismic waves. When these methods are used to determine the nature of subsurface formations, they are called seismic surveying, and when the object of the survey is to locate mineral deposits, the methods are sometimes referred to as seismic prospecting.
As currently practiced, seismic prospecting generally involves digitizing the received seismic waves and making magnetic tape recordings of digitized seismic waves and transporting or otherwise transmitting the magnetic tape recordings to a computing center where they are processed in an effort to determine the nature and structure of subsurface formations.
In the traditional method of seismic prospecting, seismic waves were generated at the surface of the earth in the form of a sharp pulse. Such waves have commonly been generated by detonation of an explosive at the bottom of a bore hole that has been drilled into the surface layers of the earth as much as several hundred feet or as little as a few feet. Sometimes the pulse has been created by the drop of a weight onto the surface of the earth or by application of an impact to the surface of earth, such as by pounding the earth with a piston or plate driven by a gas explosion.
After deflection by subsurface formations, the waves are detected at receiver points by means of electromechanical transducers, sometimes called geophones or seismometers. Sometimes they are detected by means of an array of interconnected transducers arranged in predetermined relation to a geophone station. The combined waves from these transducers are often said to be received at the geophone station.
Where an explosive in the form of a charge of dynamite has been employed, the received waves have often been sufficiently strong to enable the making of an analog record in which the undulations of the received wave are recorded as a conventional seismogram in which the amplitude of the received wave is recorded as a function of time. In order to accommodate the fact that waves reflected from deeper and deeper anomalies are of gradually decreasing average strength, amplifying systems have been employed that compensate for such gradual reduction in strength. Such systems bring the average amplitude of the wave up to a value within a range which renders the undulations of the recorded wave on the seismogram easily discernible by the eye. In early traditional seismic exploration, the records were made in the form of visible oscillograph traces. In later developments, the waves were recorded in phonographically reproducible form, either as variable density records on photographic film or on magnetic tapes or the like and were then translated into oscillographic traces, with or without prior filtering. It is therefore necessary to distinguish between phonographically reproducible seismograms and oscillographic seismograms. Still later, the received waves were sampled intermittently and the amplitudes of the successive samples were digitized, that is, translated into binary digital signals that represent the amplitudes of the samples. A record of such digital signals constitutes a digitized seismogram. Digitized seismograms are also reproducible as oscillographic seismograms. In a certain sense, therefore, the digitized seismograms are also phonographically reproducible.
It is well known that there is a background of extraneous seismic noise which is received along with the deflected waves that are recorded. This noise has its origin in wind, traffic, and even in animal movements at or beneath the surface of the earth. Often, when earth impact methods and weight dropping methods are employed, the strength of the received reflections compared with the background noise from a single weight-drop or othr impact, is lower than when a large charge of explosive is employed. In other words, such methods generally produce a lower ratio of signal strength compared with the strength of background noise. This is also the case where a small charge of explosive is used in generating the seismic impulse. For this reason, records are often made involving the repeated dropping of a weight or the repeated impacting of the earth at about the same point, thus producing a series of records of waves that have traveled over about the same paths to the geophones each time the siesmic waves are generated. In order to increase the signal-to-noise ratio and hence to make it possible to detect waves deflected from subsurface formations, especially anomalies at great depths, the various trains of waves that have traveled over approximately the same paths are combined, such as by adding, in order to increase the signal-to-noise ratio. Such a method is sometimes called vertical compositing or stacking. In order to distinguish such composing from another type of compositing which will be mentioned, we will sometimes apply the name equal-time composing to this vertical compositing. In some methods, the weight is not dropped at the same point and the earth is not impacted at the same point, so that even though the geophones have remained stationary from one weight-drop or impact to another, the waves travel over slightly different paths from one impact to another. Where the differences in the lengths of the paths are small compared with the wavelengths of the waves, no account needs to be taken of the differences in lengths of the paths in the stacking process.
In still another method of compositing, waves that have been reflected from approximately the same point beneath the surface of the earth but which have been generated at different points and also received at different points, are combined. A method for combining waves that have been so generated and recorded, is described in the Mayne U.S. Pat. No. 2,732,906. In this case, the times of travel from the source to a point of reflection and thence to the receiver are unequal. When such waves are combined, the process is often called common-depth-point stacking or compositing. In such process, account is taken of the differences in travel times over paths of different lengths by involving reflections from the same depth. The result is what we might call unequal-time compositing.
In another method of seismic prospecting as currently conducted, a train of seismic waves in the form of a "chirp" signal is generated in place of a sharp pulse, at the source.
A chirp signal is a frequency modulated wave of substantial duration in which the frequency is varied as a monotonic function of time, usually a linear function, over a substantial period of as much as several seconds. The length of a chirp is to be contrasted with a duration of a single seismic impulse produced by the methods described above, wherein the total duration of the signal emitted from the source is only a small fraction of a second, such as about 0.05 sec. or less. Sometimes a chirp signal is referred to as a sweep since the frequency is swept from one value to another during each action.
As the chirp signal travels downwardly in the earth, various parts of the signal are received simultaneously by the same geophones at the surface after reflection from anomalies that lie at different depths. Thus, at any specific instant, the frequencies of the respective signals received at the same geophone from anomalies at different depths are different.
When a chirp source signal is employed, the resultant recorded waves are cross-correlated with a replica of the original chirp in order to produce a record somewhat similar to that which would be produced if the original seismic wave had been in the form of an impulse. To achieve such a result, the received seismic waves are conventionally recorded on a magnetic tape along with a record of the chirp signal and the recorded seismic waves are correlated with the record of the chirp signal at a computer center. In order to increase the signal-to-noise ratio, a chirp signal may be produced 20 to 100 times at about the same point and received at the same receiver point and the resultant received waves subjected to equaltime compositing. The most commonly used seismic prospecting process involving the use of a chirp signal and the correlation of the chirp signal with the seismic records is known as "Vibroseis" (trademark of Continental Oil Company). For convenience, we will refer to such processes as a Vibroseis-type process.